This invention relates to a demodulation circuit of a frequency-modulated (FM) stereo receiver, and more particularly to a demodulation circuit of an FM stereo receiver of the pilot tone type in which FM stereo input composite signals are demodulated by a switching circuit.
A prior art demodulation circuit of the type referred to above is constructed to separate left and right signals from a composite signal and to remove a pilot tone from these signals. More particularly in the FM system, a signal having a frequency of 19 kHz is used as the pilot signal and this frequency is contained in an audio frequency band of from 20 to 20,000 Hz which is necessary for the stereo signal so that it is impossible to regenerate the stereo signal at high fidelity unless the pilot signal is removed.
A system for eliminating the pilot signal contained in the composite signal has been proposed as disclosed in Japanese patent application laid open specification No. 89302 of 1976 published on Aug. 5, 1976. In the demodulation circuit disclosed therein, the level of the pilot signal contained in the composite signal is detected, and the pilot signal is eliminated from the composite signal by a rectangular wave signal or a sine wave signal which is synchronized with the pilot signal and proportional to the detected level. The composite signal from which the pilot signal has been eliminated is used to demodulate the stereo signal. Although this circuit can substantially completely eliminate the pilot signal from the composite signal it is necessary to provide an elimination circuit which synthesizes the composite signal and an elimination signal on the input side of the switching circuit. However, the composite signal and the elimination signal cannot be synthesized in a satisfactory manner unless the impedance on the input side of the switching circuit is lower than a certain limit. Unless this condition is met, the pilot signal is not completely eliminated, and performance of the receiver suffers accordingly. Moreover, noise is picked up and transmitted to the switching circuit thus affecting the separation at the time of demodulating the composite signal. This also lowers the S/N ratio of the demodulated left and right signals, with further attendant deterioration in performance.